In the development of the wet blasting method and with the automation of such method, one of the desired requirements of such method is the ability to repetitively finish a series of parts by blasting while ensuring that the blasting is maintained with the same degree of uniformity or equality so as to ensure that the process can be repeated with uniformity and consistency. To achieve this objective, the ratio of water and abrasive particles, that is, the concentration of the slurry, should be maintained within a predetermined range.
To provide a machine which attempts to meet the above objectives, the assignee of this invention has developed a machine which attempts to maintain the slurry concentration, such machine being illustrated by Japanese Pat. No. 55-8311. This prior machine, which is schematically illustrated in FIG. 2 hereof, has a device D for detecting the slurry concentration within the slurry supply line which connects the slurry supply hopper a.sub.1 with the slurry blasting gun a.sub.2. Depending upon the result of the measurement by this device D, the fluid control valve G which is provided in the pipe F automatically operates and supplies liquid from the top layer of the slurry settling tank E to the outlet a.sub.4 of the slurry supply hopper a.sub.1. In this manner, an attempt is made to prevent the slurry from becoming too heavy so as to keep the slurry concentration in a prescribed range as the slurry is supplied by the pump P.sub.1 to the blasting nozzle. This arrangement also facilitates the start of the machine, but is not capable of maintaining slurry concentration within a more desirable range when the slurry becomes light in weight.
One of the primary problems associated with controlling the slurry concentration is the difficulty in measuring the content of the abrasive particles within the water as the slurry is supplied to the blasting gun. Generally, the measurement of the slurry concentration can not be optimally performed by installation of measuring tools within the flow line inasmuch as the slurry contains the abrasive particles. If the viscosity of the slurry is equal or substantially equal to that of water, and the flow rate of the slurry is constant, then the concentration of the slurry can be obtained by measuring the dynamic flow pressure of the slurry. In this case, however, the abrasive nature of the slurry and its flow through the pipe to the blasting gun causes substantial fluctuations and changes according to whether the gun is operating or not, so that measurement of dynamic pressure of flow is extremely difficult.
In an attempt to avoid the above problem, another method attempts to detect slurry concentration from outside the pipeline. In this measuring method, such as disclosed in the assignee's aforementioned patent, a measuring apparatus which is either supersonic or which involves use of an electro-magnetic flow meter is involved. Although such devices have a stabilized function, nevertheless these devices are of a complicated structure and such devices are also expensive.
Accordingly, it is an object of this invention to provide a slurry concentration measuring arrangement for use with a wet blasting apparatus, which measuring apparatus overcomes the defects and disadvantages noted above. More specifically, this invention provides an automatic slurry concentration measurement which is not affected by the flow rate of the slurry or the abrasive nature thereof, and which possesses a simple structure and provides consistent results.
In the improved wet blasting machine of this invention, there is provided a slurry concentration control device involving a vessel for holding a given volume of slurry, and a bleed line is connected between the vessel and the main slurry supply line as connected to the blasting gun. The bleed line permits filling of the vessel with slurry, and a measuring device is associated with the vessel to create an electrical signal which is a function of the weight of slurry in the vessel. This signal representing the measured quantity of slurry is compared to a predetermined signal which represents the desired slurry concentration. If the measured value indicates that the actual concentration is less than the desired amount, then a signal activates an abrasive make-up system so that some slurry is withdrawn from the main tank into a make-up tank, and additional abrasive is supplied to this make-up tank for mixing with the slurry therein. A time-delayed valve is then opened so that the concentrated slurry in the make-up tank is supplied to the main tank so as to increase the concentration of the abrasive particles therein.
Other objects and purposes of the invention will be apparent to persons familiar with systems of this general type upon reading the following specification and inspecting the accompanying drawings.